1. Field of the Invention
The invention relates to a process for preparing triethyl phosphate by reacting phosphorus oxychloride with ethanol.
2. Brief Description of the Prior Art
Trialkyl phosphates have a broad application spectrum. They serve, for example, as plasticizers, unreactive flame retardants, hardeners and accelerants in plastics and coatings, and are used as auxiliaries in textiles and paper. They also find use in the chemical industry as wetting agents, flotation agents, defoamers, emulsifiers, stabilizers or as extractants.
A variety of processes for preparing trialkyl phosphates from phosphorus oxychloride and the corresponding alcohols are known. In the past, attempts were made to improve the process with regard to the reaction temperature, ratio of the components used and removal of the heat of reaction. A problem in the reaction of these two components is that hydrogen chloride, which is formed has to be removed from the reaction mixture very quickly and completely, in order to avoid acidolytic cleavage of the esters formed.
DE 541 145 C describes a process for preparing phosphate esters of aliphatic alcohols which react phosphorus oxychloride with the corresponding alcohols at reduced pressure and elevated temperature, so that hydrogen chloride is continually removed from the reaction mixture. A disadvantage of the application of this process for preparing triethyl phosphate is that it leads to considerable losses of ethanol, which is likewise removed from the reaction mixture under the reaction conditions.
DE 899 498 C and U.S. Pat. No. 2,636,048 disclose the removal of hydrogen chloride from the reaction mixture by passing in inert gas. This measure likewise leads disadvantageously to considerable losses of ethanol when preparing triethyl phosphate.
The abstract of KR 9501703 A discloses the use of both solvents which only dissolve hydrogen chloride to a small extent and also of bases which neutralize hydrogen chloride in the reaction mixture. A disadvantage thereof is that the use of solvents is associated with a reduced space-time yield. Although the use of bases avoids acidolytic cleavage of the reaction products, neutralization of the reaction mixture, in addition to the costs of the neutralizing agent, also results in costs associated with the resulting wastewater and the workup thereof, which make this process variant unattractive for industrial scale preparation.
In preparing triethyl phosphate, the workup is particularly difficult owing to the complete miscibility of ethanol and triethyl phosphate with water, since ethanol has to be distilled out of the aqueous phase and triethyl phosphate has to be extracted from the aqueous phase and then isolated. This results in undesirable losses of product, ethanol and extractant.
There is accordingly a need for an improved process for preparing triethyl phosphate which, with regard to process economy and ecology, meets at the same time the requirements for industrial scale preparation by:
a) removing the hydrogen chloride in such a manner that there is ideally no acidolysis of triethyl phosphate, no loss of ethanol and no additional cost due to the use or disposal of additional materials, such as bases or extractants,
b) removing the heat of reaction in an efficient manner and
c) keeping the ethanol excess as small as possible.
Surprisingly, there has now been found a process for preparing triethyl phosphate wherein
a) phosphorus oxychloride is reacted with a greater than stoichiometric quantity of ethanol under reduced pressure at temperatures of from 0 to 50xc2x0 C. in a reaction vessel, and the volatile components resulting from the reaction are predominantly condensed by means of a reflux condenser and the remaining volatile components are passed into a scrubber filled with water,
b) after the end of the reaction, the reaction mixture is separated distillatively in a distillation column, referred to hereinbelow as an outgassing column, into a top product and a bottom product which predominantly comprises triethyl phosphate,
c) the top product of the outgassing column is combined with the contents of the scrubber and
d) the contents of the scrubber are subjected to a distillation, referred to hereinbelow as an azeotropic distillation, to obtain water and ethanol as top product and the ethanol, preferably after dewatering, is preferably returned to the reaction.
The process according to the invention enables the heat of reaction to be removed effectively and economically, since ethanol and hydrogen chloride boil under the conditions mentioned, so that volatile components arise in the form of a mixture of ethanol and hydrogen chloride, and the heat of reaction is quickly and effectively removed by evaporative cooling. The reflux from the reflux condenser also contributes advantageously to cooling of the reaction vessel.
The process according to the invention also enables the concentration of hydrogen chloride in the reaction mixture to be minimized. The loss of ethanol is likewise minimized by the process according to the invention. The portion of the gaseous ethanol which is not condensed by the reflux condenser and does not drop back into the reaction mixture is intercepted in the scrubber and recovered by distillative separation of the contents of the scrubber. The process according to the invention also enables the excess of ethanol which has to be used in the process to be reduced.
The process according to the invention can be implemented industrially in a simple and advantageous manner and further leads to triethyl phosphate in high yields and purities.
The invention is described more fully hereunder with particular reference to its preferred embodiments. Preference is given to using the reactants phosphorus oxytrichloride and ethanol in the process according to the invention in technical grade purity. Particular preference is given to using highly pure products, since this enables the best yields to be achieved. Very particular preference is given to using ethanol having a very low water content, preferably from 0 to 0.3% by weight of water, more preferably from 0 to 0.1% by weight of water.
In the process according to the invention, a greater than stoichiometric quantity of ethanol is used, based on phosphorus oxytrichloride. Preference is given to using from 3 to 15 mol of ethanol per mole of phosphorus oxytrichloride, more preferably from 4 to 10 mol, and most preferably from 4.5 to 6.5 mol.
Step a) of the process according to the invention is carried out under reduced pressure, preferably at a pressure of from 30 to 600 mbar, more preferably at a pressure of from 60 to 250 mbar, and most preferably at a pressure of from 80 to 150 mbar.
Preference is given to operating the reflux condenser in the process according to the invention at temperatures of from xe2x88x9250 to 0xc2x0 C., preferably from xe2x88x9240 to xe2x88x925xc2x0 C., and more preferably from xe2x88x9230- to xe2x88x9210xc2x0 C.
The temperature in step a) of the process according to the invention is from 0 to 50xc2x0 C., preferably from 5 to 50xc2x0 C., and more preferably from 10 to 30xc2x0 C.
The reaction vessel used in step a) of the process according to the invention is a customary reaction vessel known to those skilled in the art which is suitable for the reaction conditions mentioned. Preference is given to providing the reaction vessels with additional external cooling. Preference is also given to providing them with a stirring device. Preference is given to using a multistage reaction vessel, more preferably with a stirrer battery which consists of individual stirred vessels. In such multistage reaction vessels, preference is given to increasing the reaction temperature in each further reaction vessel in stages, in such a way that the temperature in the last reaction vessel is preferably from 25 to 35xc2x0 C. This results in a commercially viable reaction time which is typically from about 6-8 h.
The volatile components arising in step a) in the process according to the invention contain predominantly of ethanol and hydrogen chloride.
A substantial portion of these volatile components condenses on the reflux condenser and contributes advantageously to the cooling of the reaction vessel by the reflux into the reaction mixture. The portion of the volatile constituents which is not condensed on the reflux condenser is passed into the water-filled scrubber. Preference is given to effecting this transfer via a butterfly valve, since this enables the desired pressure in the reaction vessel to be regulated.
After the end of step a) of the process according to the invention, preference is given to freeing the reaction mixture of volatile components in the outgassing column in step b) immediately after step a).
Preference is given to carrying out the distillation in the outgassing column in step b) at a pressure of from 10 to 100 mbar, more preferably from 15 to 60 mbar, and most preferably from 20 to 40 mbar, which results in a bottom temperature of from about 105 to 115xc2x0 C. The bottom product obtained from the distillation contains predominantly triethyl phosphate, preferably of over 95% by weight of triethyl phosphate.
In a preferred form of the process according to the invention, the triethyl phosphate obtained is further purified by distillation. Preference is given to effecting this distillation at a pressure of from 2 to 50 mbar, more preferably from 5 to 20 mbar, and most preferably from 10 to 15 mbar. Triethyl phosphate which preferably has a purity of more than 99% by weight is obtained at the top at temperatures of from about 80 to 90xc2x0 C. The bottom product obtained is predominantly diethyl phosphate.
Preference is given to condensing the top product obtained from the distillation in the outgassing column in step b) of the process according to the invention which contains predominantly of a mixture of ethanol and hydrogen chloride. Preference is given to carrying out the condensation at temperatures of from xe2x88x9240 to xe2x88x9210xc2x0 C., and more preferably at from xe2x88x9225 to xe2x88x9217xc2x0 C. This condensation step results in a top product which contains a condensate and of volatile components. The condensate is combined with the contents of the scrubber, preferably without intermediate storage. Preference is given to combining it with the contents of the scrubber by passing the condensate into the effluent of the scrubber. The combination of the condensate with the contents of the scrubber almost completely suppresses the undesired reaction of ethanol with hydrogen chloride to form ethyl chloride. The volatile components of the top product are likewise combined with the contents of the scrubber, and preference is given to passing them into the water-filled scrubber.
The scrubber used in the process according to the invention is filled with water. Preference is given to operating it at a temperature of from 5 to 20xc2x0 C., and greater preference to operating it at from 7 to 15xc2x0 C. The pressure in the scrubber is preferably from 10 to 100 mbar, and more preferably from 15 to 25 mbar.
The scrubber used in the process according to the invention preferably has a device for feeding in fresh water which is preferably located at the top of the scrubber. Preference is given to feeding in such a quantity of fresh water that the composition in the liquid phase of the scrubber, combined with the condensate of the top product from step b) of the process according to the invention gives a mixture which, as well as ethanol, also contains from 80 to 90 parts by weight of water, based on from 10 to 20 parts by weight of hydrogen chloride. Greater preference is given to adding such a quantity of fresh water that the distillative separation of the contents of the scrubber effected in step d) of the process according to the invention gives a top stream which has a composition very close to the azeotropic mixture of ethanol and water, and a bottom stream which has a composition very close to the azeotropic mixture of hydrogen chloride and water. The scrubber preferably has an outlet in its lower section, preferably in the form of a scrubbing circuit. In a preferred form, the outlet is attached to a distillation column which serves to distillatively separate the contents of the scrubber which are discharged via the outlet.
In step d) of the process according to the invention, the contents of the scrubber are separated distillatively in the azeotropic distillation into a top product which comprises predominantly ethanol and a small proportion of water, and a bottom product which comprises predominantly water and a small proportion of hydrogen chloride. The top product preferably comprises from 80 to 96% by weight of ethanol and from 4 to 20% by weight of water and the bottom product preferably comprises from 80 to 90% by weight of water and from 10 to 20% by weight of hydrogen chloride.
In a preferred embodiment, the azeotropic distillation in step d) is carried out at atmospheric pressure, and a top product which comprises about 90% by weight of ethanol is withdrawn at the top at about 75xc2x0 C. and the bottom stream boils at about 110xc2x0 C. and comprises about 18% by weight of hydrogen chloride.
The aqueous ethanol obtained as top product can be dewatered by methods known to those skilled in the art. Preference is given to carrying out the dewatering using glycol. In a preferred embodiment of the process according to the invention, the dewatered ethanol is returned back into the reaction a).
In a preferred embodiment, the process according to the invention is carried out in whole or in part by a continuous method. The reaction vessels used in the continuous method are preferably tube reactors, reaction loops, reaction columns or reaction batteries.
The following non-limiting examples illustrate the process according to the invention.